Crosslinking is one of the most widely used chemical reactions in polymer science. Linking individual polymer chains together via formation of (non)covalent bonds often imparts better mechanical properties to the polymeric material. A wide range of chemistries have been used for crosslinking purposes. These can be broadly classified as thermal stimuli based, photochemistry based, catalyst based, among others.
Benzocyclobutenes (BCBs) are one class of crosslinkers that have traditionally attracted attention because they can be thermally activated to undergo ring opening isomerization forming highly reactive intermediates that react to form C—C bonds leading to stable crosslinked structures. BCB's offer other advantages such as no formation of byproducts and no need for a catalyst. Nevertheless, the high temperature required for ring opening limits their uses in many systems. One of the drawbacks of BCBs is that they require very high temperatures (>200° C.) to undergo ring opening isomerization followed by crosslinking. These temperatures can be reduced by introducing either electron donating and/or electron withdrawing substituent at the benzylic carbon(s). Harth and coworkers have reported a BCB-based crosslinker that undergoes ring opening at a lower temperature but their approach is based on post-polymerization attachment of a crosslinker to the polymer backbone, while at least some embodiments of the invention disclosed herein provide for direct incorporation of BCB-based crosslinker into the polymer during polymerization. Harth, E. et. al., Polym. Chem. 2012, 3, 857-860.
Benzocyclobutene (BCB), a bicyclic compound with a 4-membered ring, has been widely used for its ability to undergo thermal ring opening isomerization to form a highly reactive intermediate, o-quinodimethane (oQDM). In the presence of a dienophile it undergoes cycloaddition reaction to form the corresponding cycloadduct whereas in the absence of a dienophile reacts irreversibly with itself forming dibenzocyclooctadiene or oligomeric structures containing C—C bonds. This is shown in FIG. 1, reaction scheme (a), which shows thermal ring opening isomerization of 1-substituted BCBs to form corresponding oQDMs and subsequent products. Temperatures mentioned for various substituents are approximate temperatures based on literature; such as: Oppolzer, W. Synthesis 1978, 793-802; Chino, K. et al.; Macromolecules 1997, 30, 6715-6720; and Chino, K. et al.; J. Polym. Sci.: Part A: Polym. Chem. 1999, 37, 1555-1563.
Choy and coworkers have reported a BCB with alkoxide anion as substituent on the benzylic carbon that undergoes ring opening and cycloaddition reactions with dienophiles at temperatures as low as −78° C.-0° C., as shown in FIG. 1, reaction scheme (b). Choy, W.; Yang, H., “Diels-Alder Reactions of α-oxy-o-xylylenes”, J. Org. Chem. 1988, 53, 5796-5798. This accelerated ring opening of BCBs using anions has been rarely used in organic chemistry, and has not been used in polymer chemistry. Choy, W.; Yang, H., “Diels-Alder Reactions of α-oxy-o-xylylenes”, J. Org. Chem. 1988, 53, 5796-5798. Shaw, S. J., “Ring-Opening of Benzocyclobutenol with Mild Bases and Trapping with Dienophiles”, Synthetic Commun. 2007, 37, 4183-4189. There is a need in the art for BCBs giving access to polymers that can be crosslinked at ambient or even sub-ambient temperatures. Herein, ambient or room temperature is to be understood as being from 60 to 95° F. (15.5 to 35° C.).
The ring opening isomerization of BCB to oQDM is a temperature dependent equilibrium, i.e., at any given temperature, there will be a distribution of unreacted BCBs and products formed from reacted BCBs at that temperature. As the temperature increases, half-life of oQDM decreases leading to faster and higher consumption of BCBs to form cycloadducts. This temperature-dependent equilibrium of BCBs can be useful in controlling the crosslinking of BCB-containing polymers. For example, a BCB-containing polymer could be crosslinked slower at lower temperature or could be crosslinked faster at higher temperature. This ring opening temperature can be lowered by substituting one or more benzylic positions of a BCB by a substituent. Both the electron donating as well as withdrawing groups have been shown to lower the ring opening isomerization temperature. Segura, J. L.; Martin, N., “O-quinodimethanes: Efficient Intermediates in Organic Synthesis”, Chem. Rev. 1999, 99, 3199-3246. The effect of a substituent in lowering the ring opening isomerization is noticeable even in the case of mono-substituted BCBs (1-substituted BCBs) as shown in FIG. 1, scheme (a). It is worth noting that the temperature dependent equilibrium is also valid in the case of 1-substituted BCBs thereby availing the ability to control crosslinking parameters in 1-substituted BCB containing polymers. Oppolzer, W. Synthesis 1978, 793-802, has reported the approximate reaction temperatures (for 18 hour reactions) for various 1-substituted BCBs, whereas Chino, K. et al.; Macromolecules 1997, 30, 6715-6720 and Chino, K. et al.; J. Polym. Sci.: Part A: Polym. Chem. 1999, 37, 1555-1563 has reported examples of electron donating and electron withdrawing group as a substituents.
Though thermal activation of BCBs for the formation of crosslinked material has gained wide attention in the polymer field, chemical activation of BCBs has remained neglected. It has been rarely used in organic chemistry for the synthesis of intermediates. This disclosure reports the synthesis of a new, polymerizable, BCB-based crosslinkers having thermally stable 4-membered ring yet the copolymer containing this BCB can be chemically activated to undergo rapid crosslinking at room temperature using a suitable nucleophile. In some embodiments, this rapid crosslinking at room temperature has been utilized to make single chain polymer nanoparticles via intramolecular chain collapse utilizing what is termed herein a pseudo-high dilution continuous addition method.